MAZDA 3 TIRE PROBLEM

2006 Mazda 3 • 136,000 miles

My car shudders at 45mph and then stops at 50mph, but only in the automatic transmission setting, not in the manual setting. It shakes around right hand turns above 20mph. The primary area of the shaking is the steering wheel. You can feel it in the floor boards nut not much. The tansmission fluid, oil, and coolant are new so are the spark plugs. The front tires are old and need replacing. The rear tires are new. The front right side suspension looks crudy, the left does not. The # 3 and 4 motor mounts look fine. The car when cold idles at 1,500 to 2,000 rpms. When warm it idles at 600. The a/c makes the revs go up and down and the car rattles. This is everything I can think of. Thank you for your help.